投稿者: uemuram

Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars

M. Uemura, K. ~S. Kawabata, M. Sasada, Y. Ikejiri, K. Sakimoto, et al., “Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars,” PASJ, vol. 62, p. 69-, 2010.

We developed a method of separating a long-term trend from observed temporal variations of polarization in blazars using a Bayesian approach. The temporal variation of the polarization vector is apparently erratic in most blazars, while several objects have occasionally exhibited systematic variations, for example an increase of the polarization degree associated with a flare of the total flux. We assume that the observed polarization vector is a superposition of two distinct components: a long-term trend and a short-term variation component responsible for short flares. Our Bayesian model estimates a long-term trend that satisfies the condition that the total flux correlates with the polarized flux of the short-term component. We demonstrate that assumed long-term polarization components are successfully separated by the Bayesian model for artificial data. We applied this method to the three photopolarimetric data of OJ 287, S5 0716+714, and S2 0109+224. Simple and systematic long-term trends were obtained in OJ 287 and S2 0109+224, while no such trend was confirmed in S5 0716+714. We propose that the apparently erratic variations of polarization in OJ 287 and S2 0109+224 are due to the presence of the long-term polarization component. The behavior of polarization in S5 0716+714 during the period of observation implies the presence of a number of polarization components showing variations on a quite short time-scale.

uem10blazar

 

 

Optical and Near-Infrared Photometric Observation during the Superoutburst of the WZ Sge-Type Dwarf Nova, V455 Andromedae

R. Matsui, M. Uemura, A. Arai, M. Sasada, T. Ohsugi, et al., “Optical and Near-Infrared Photometric Observation during the Superoutburst of the WZ Sge-Type Dwarf Nova, V455 Andromedae,” PASJ, vol. 61, p. 1081, 2009.

We report on optical and infrared photometric observations of a WZ Sge-type dwarf nova, V455 And during a superoutburst in 2007. These observations were performed with the KANATA (V, J, and Ks bands) and MITSuME (g’, RC, and IC bands) telescopes. Our 6-band simultaneous observations allowed us to investigate the temporal variation of the temperature and the size of the emitting region associated with the superoutburst and short-term modulations, such as early and ordinary superhumps. A hot (>11000 K) accretion disk suddenly disappeared when the superoutburst finished, while blackbody emission, probably from the disk, still remained dominant in the optical region with a moderately high temperature (˜8000 K). This indicates that a substantial amount of gas was stored in the disk even after the outburst. This remnant matter may be a sign of an expected mass-reservoir, which can trigger echo outbursts observed in several WZ Sge stars. The color variation associated with superhumps indicates that viscous heating in a superhump source stopped on the way to the superhump maximum, and a subsequent expansion of a low-temperature region made the maximum. The color variation of early superhumps was totally different from that of superhumps: the object was bluest at the early superhump minimum. The temperature of the early superhump light source was lower than that of an underlying component, indicating that the early superhump light source was a vertically expanded low-temperature region at the outermost part of the disk.

mat09v455

Optical Observation of the 2003 Outburst of a Black Hole X-Ray Binary, V4641 Sagittarii

M. Uemura, T. Kato, R. Ishioka, A. Imada, D. Nogami, et al., “Optical Observation of the 2003 Outburst of a Black Hole X-Ray Binary, V4641 Sagittarii,” PASJ, vol. 56, pp. 823-829, 2004.

We report on the results of our optical photometric observations of a black hole X-ray binary, V4641 Sgr during an outburst in 2003 August. During the outburst, we detected several properties having noteworthy similarities to those observed during an outburst in 2002 May; the outburst lasted about 6 days, during which it exhibited flares and rapid fluctuations, having timescales of 102-4s. In complicated profiles of light curves, we identified several recurring patterns, which were also observed during the 2002 outburst. First, the object frequently experienced sudden temporary fadings, which we call “dips”, appearing just after the states characterized by strong short-term fluctuations. Second, we detected optical flashes, which are characterized by a brightening by ˜ 0.5 mag within ˜ 50 s. Based on the similarity of the outbursts in 2002 and 2003, we conclude that they have the same nature. This is the first time that repetitive outburst phenomena have been confirmed in V4641 Sgr. The object probably has an outburst interval that can be as small as 1-2 years.

uem04v4641

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